The present invention relates generally to a method and apparatus for finding shape deformations in objects having smooth surfaces and in particular, to a method and apparatus for finding shape deformations in airfoils by registering surface points on an airfoil to a computer assisted drawing (CAD) model and analyzing and displaying principal modes of surface deformation to a user.
Currently, automated inspection is performed on manufactured parts such as airfoils (e.g. turbine blades) to detect deformities in the part. For purposes of this specification, deformities are defined to be deformations in the shape of an object as compared to its ideal shape. In other words, deformations are deviations in shape or form of an object from its manufacturing specifications. Shape deformation can occur in a variety of modes (referred to herein as deformation modes). Deformation modes include, but are not limited to, skew, twist, scaling and translation. Forged blades, such as those for aircraft engines, are currently inspected for platform orientation, contour cross-section, bow and twist along stacking axis, thickness and chord length at given cross-sections. One method of inspecting for shape deformations in these deformation modes is through the use of specialized hard gages built out from micro-meters, calipers, and shims. The hard gages measure a plurality of defined contact points to characterize the blade that is used to detect defects. While the inspection of using the hard gages is fast, the hard gages provide only individual measurements at a few defined contact points.
In the alternative, blades can be fully scanned with coordinate measurement machines (commonly know as xe2x80x9cCMMsxe2x80x9d) that translate and rotate a probe to sample points on the blade surface. CMMs provide dense measurements of the sample points, however the time to scan a blade is relatively slow. Once the dense surface points are collected, software processes these points into deviations to the CAD model and analyzes the deviations in terms of process-based shape deformations. Current processing software, however, is relatively slow.
Full-field non-contact range sensors can scan the external surfaces of the blade at 100xc3x97 faster than CMMs. These non-contact range sensors, however, are 100xc3x97 less accurate than the CMMs. Full-field non-contact range sensors are currently commercially available and include sensors based on laser line grating and stereo triangulation; single laser line scan plus rotating the part; and on phased-shift Moire and white light.
CAD/CAM software exists, such as ValiSys, a quality assurance software product available from Tecnomatix Technologies Inc., and UniGraphics Solutions(trademark), which can register the surface points scanned by non-contact range sensors to a CAD model. The CAD/CAM software, however, has disadvantages in that it does not allow registration to special features characteristic of the shape of an airfoil, such as a 6-point nest. The points of a six point nest are: 4 points on the leading edge of the airfoil, 1 point near trailing edge and 1 point on the platform. The 6-point nest is common in airfoil analysis, design and manufacturing because it weights more accuracy on the leading edge, which is critical for the desired airflow.
The present invention is directed to an imaging apparatus for examining objects having smooth surfaces for shape deformations. The imaging apparatus includes an imaging device for obtaining a scanned image of the object to be examined. A reference image of the object is stored in a memory. An image register is coupled to the imaging device and to the memory containing the reference image of the object. The image register stores patch information corresponding to both the reference image and the scanned image. A transformation estimator compares the scanned image to the reference image, and provides a transform which maps the scanned image to the reference image. A deformation estimator is coupled to the image register and to the transformation estimator. The deformation estimator decomposes the transformation parameters, based on patch information stored in the image register to determine shape deformations of the object.